Micro-Scale Response Characteristics of Ni52Ti48 Alloy under Shock Loading
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摘要: 为了了解近等原子比NiTi合金在高压高应变率下的动态变形行为和微结构演化特性及机制，采用实验和分子动力学模拟方法，开展了NiTi冲击压缩和冲击加-卸载拉伸研究。在实验方面，基于大电流脉冲功率CQ-4装置，利用电磁驱动高速飞片，结合动量陷阱和软回收实验技术，开展了冲击压缩与冲击加-卸载拉伸作用下Ni52Ti48合金的动态变形特性研究，借助X射线衍射和电子背散射衍射显微技术，对回收Ni52Ti48合金样品进行微结构特征观察和分析。结果表明，Ni52Ti48在冲击压缩和拉伸下都没有发生马氏体相变，主要变形方式为位错滑移等塑性变形。在分子动力学数值模拟方面，计算结果很好地反映了实验观察到的微结构特征，计算得到的不同初始环境温度和不同冲击速度下Ni52Ti48合金的层裂强度表现出明显的卸载拉伸应变率效应。相关工作加深了对Ni52Ti48合金在高压高应变率下变形行为的理解和认识，为其在极端环境下的安全服役提供了参考。Abstract: Experiments and molecular dynamics simulations were carried out to understand the dynamic deformation behavior, microstructure evolution characteristics and mechanism of near equiatomic NiTi alloy at high pressures and high strain rates. In the experiments, the dynamic deformation characteristics of Ni52Ti48 alloy under shock compression and shock loading-unloading were studied by electromagnetically driven high-velocity flyer plate, momentum trapping and soft recovery experimental techniques based on high current pulse power device CQ-4. By means of X-ray diffraction and electron backscattered diffraction, the microstructure characteristics of Ni52Ti48 alloy were analyzed, the results show that there is no martensitic transformation in Ni52Ti48 alloy under shock compression and tension, and the main deformation mode is plastic deformation such as dislocation slip. Moreover, the microstructure evolution characteristics and deformation mechanism of Ni52Ti48 alloy under shock compression were studied by non-equilibrium molecular dynamics simulations, and the calculated results well reflect the microstructure characteristics observed in the experiment. Meanwhile, the spall strength of Ni52Ti48 alloy at different initial ambient temperatures was calculated, and the results show obvious unloading strain rate effect. The related work has deepened the understanding of the deformation behavior of Ni52Ti48 alloy at high pressures and high strain rates, and provided a reference for its safe service in extreme environment.
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图 5 冲击加载速度up = 0.927 km/s时冲击压缩回收NiTi样品的反极图(a)以及孪晶(b)和再结晶(c)的局部放大图
Figure 5. (a) EBSD characterizations of experimentally recovered polycrystalline NiTi samples at shock loading velocity up = 0.927 km/s at room temperature, and the corresponding amplified configurations in (a), which represent (b) twins and (c) re-crystalline, respectively
图 6 冲击压缩加载下NiTi样品在横向（x0）、纵向（y0）和法向（z0）的取向分布散点图(a)和取向分布密度图(b)
Figure 6. IPF distribution of horizontal (x0), longitudinal (y0) and normal (z0) direction of NiTi under shock compression: (a) scatter diagrams, (b) contour map
图 7 NiTi合金初始样品(a)和冲击压缩回收样品(c)的极射赤面（赤道面）投影图及其对应的晶界角度分布(b, d)
Figure 7. Pole figures (equatorial plane) of NiTi: (a) as-received and (c) recovered samples under shock compression; (b) and (d): the histograms of frequency and distribution of the boundaries for (a) and (c), respectively
图 8 不同冲击粒子速度下的一维应力波剖面演化(a)；不同冲击粒子速度下的演化结果比较：(b) 0.6 km/s，(c) 0.8 km/s，(d) 1.0 km/s（不同的状态以红色长划线区分，微结构包括孪晶T1、T2、T3和新晶粒（NG），用CNA方法进行表征，冲击方向用黑色箭头标示）；初始奥氏体以及图8(b)、图8(c)和图8(d)所示nc-NiTi模型的模拟XRD分析结果(e)[20]
Figure 8. (a) 1D pressure profiles in nc-NiTi under different shock-loading velocities; comparisons of simulated results corresponding to (a) at initial ambient temperature 300 K and different loading velocities: (b) 0.6 km/s, (c) 0.8 km/s, (d) 1.0 km/s (Different states are distinguished by red long dashes. The microstructures are characterized by CNA methods: twin T1, T2, T3 and new grain (NG). The shock direction is labelled by black arrows.); the simulated XRD patterns (e) of the nc-NiTi models for initial austenite, Fig.8(b), Fig.8(c) and Fig.8(d), respectively[20]
图 11 up = 1.0 km/s时三叉晶界处形成的非晶剪切带以及B2结构的sc-NiTi和nc-NiTi中非晶剪切带的径向分布函数g(r)[20]
Figure 11. Formation of amorphous shear band at grain boundaries (GBs) triple junction for shock loading velocity up = 1.0 km/s, and the radical distribution function g(r) of the B2 structure sc-NiTi and amorphous shear band in nc-NiTi[20]
图 12 (a)～(c) up = 0.8 km/s时新晶粒的微结构演变，(d)～(f) 微结构演变沿x轴的OM分析结果，(g)～(h) 变形前后B2-NiTi在(011)面上的投影[20]
Figure 12. (a)–(c) Microstructural evolution of new grain for shock loading velocity up = 0.8 km/s; (d)–(f) the corresponding region based on OM analysis along the x axis; projection of B2-NiTi crystal position on (011) plane before (g) and after (h) deformation[20]
图 13 冲击拉伸实验回收样品的EBSD 表征分析结果：(a)～(c) Shot 741, (d)～(f) Shot 739, (g)～(h) Shot 738
Figure 13. EBSD characterization analysis results of samples recovered from shock tensile experiments: (a)–(c) Shot 741, (d)–(f) Shot 739, (g)–(h) Shot 738
图 14 NiTi合金冲击拉伸实验回收样品的极射赤面（赤道面）投影图：(a) Shot 741，
$\sigma $ H = 6.4 GPa；(b) Shot 738，$\sigma $ H = 12.4 GPaFigure 14. Pole figures (equatorial plane) of the NiTi alloy recovered from shock tensile experiments: (a) Shot 741,
$\sigma $ H = 6.4 GPa; (b) Shot 738,$\sigma $ H = 12.4 GPa
图 15 对应于图13中EBSD表征结果的局部变形程度以及变形结构、再结晶结构和亚结构分布：(a)～(b) Shot 739，
$\sigma $ H = 8.5 GPa；(c)～(d) Shot 738，$\sigma $ H = 12.4 GPaFigure 15. Local deformation degree and distribution of the deformed structure, recrystallized structure, and substructure of the EBSD characterization in Fig.13: (a)–(b) Shot 739,
$\sigma $ H = 8.5 GPa; (c)–(d) Shot 738,$\sigma $ H = 12.4 GPa
图 16 不同初始环境温度及不同冲击速度下的自由面速度历史
Figure 16. Free surface velocity histories corresponding to different shock loading velocities and initial ambient temperatures
图 17 不同应变率下NiTi合金的层裂强度
Figure 17. Spall strength of NiTi alloys at different strain rates
Composition $\,\rho $/(g·cm−3) CLO/(km·s−1) Cs/(km·s−1) Cb/(km·s−1) $\nu $ TMs/℃ TMf/℃ TAs/℃ TAf/℃ Ti46–48Ni52 6.42 5.434 1.775 5.032 0.436 −14.6 −19.7 −11.4 −0.7 
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表 2 不同初始环境温度下的层裂强度和拉伸应变率
Table 2. Spall strength and tensile strain rate at different initial ambient temperatures
T/K up/(km·s−1) Binning analysis Acoustic approximation $\sigma{ _{ {\rm{sp} } }^{ {\rm{MD} } } }$/GPa ${\dot \sigma {_{ {\rm{MD} } }} }$/(1010 s−1) $\sigma {_{ {\rm{sp} } }^{\rm{a} }}$/GPa ${\dot \sigma {_{\rm{a} }} }$/(1010 s−1) 300 0.2 0.4 9.4 0.7 11.0 0.6 0.6 10.5 1.4 11.2 0.6 0.8 10.5 1.9 11.0 0.7 500 0.2 0.4 9.0 1.0 9.5 1.0 0.6 10.0 2.0 10.6 1.1 0.8 9.9 2.5 10.4 1.1 1000 0.2 0.4 8.1 0.8 8.7 0.7 0.6 8.9 1.3 9.5 1.0 0.8 8.3 2.8 9.0 1.3
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